An Introduction to Embedded Linux Development, Part 2

Part 2 in a series on embedded development explains how to establish serial communication between an LBox and a workstation, compile tool chains and write and run a simple program.

In
Part 1 of this series, we indicated that we would use,
as our SBC, the LBox with uClinux from
Engineering Technologies Canada Ltd.. Recall that
it features the Motorola Coldfire MCF5272 processor,
Flash memory, a serial port, a fiber port and up to three
10/100 Ethernet ports. It's ready to go without needing to be built
into something else--simply power it up with
any suitable power supply in the 5-12 volt range.
Although we are using a specific SBC for this project, the activities we undertake
here correspond to similar activities on any typical SBC.
That said, significant specific differences exist
at the more detailed level from one SBC to another.

I purchased about 12 of these systems for our computer
science department. If not purchased in quantity, the basic
board goes for about $250. Then, you add whatever else you
need.

Following along with this series while using an actual LBox SBC
would be optimal. Nevertheless, I have organized this series
of articles so a reader can glean useful information
without purchasing the board. Yet another option would be
to use some other SBC and parallel our activities.

To avoid putting forth too much nitty gritty detail here, I
refer you to information posted in the FAQ section of the Engineering Technologies
Web site.

The goals and subsequent sections for the current article
are:

Power up the LBox.

Establish serial communication between LBox and
workstation, including what to do if your workstation has no
serial port.

Connect via Ethernet.

Install the cross compiling tool
chains.

Carry out NFS mounting.

Write a program for the LBox and run
it.

The last two sections are quite general and apply to most embedded Linux systems.

Power Up the LBox

My particular setup consists of:

the LBox SBC (from Engtech)

a power supply (from Engtech)

a serial header-to-DB9 cable (from
Engtech)

a CD with all needed software (from
Engtech)

my laptop (the workstation) with Libranet 2.81, updated to
the 2.4.27 kernel

a Belkin F5U409 USB-to-DB9 adapter because my laptop has
no external RS-232 DB9 port but does have USB ports

I configured the laptop to use the widely available, tried
and true Minicom terminal emulator for the serial
connection. It comes with most Linux distributions, and for
connecting to SBCs with serial ports, Minicom is a common
choice. The Belkin F5U409 uses the mct_u232 driver, which
is available with the kernel source. It didn't
work properly for me with the 2.4.24 kernel, however, hence the
update to 2.4.27.

Before applying power, I connected an Ethernet cable and
the serial cable. The Ethernet ports provided on the LBox,
when populated, have the expected RJ45 female sockets. The
serial port header allows connection of the serial
header-to-DB9 cable, which I connected to my laptop via the
Belkin F5U409. At this point, everything seemed ready, so I
applied power by plugging in the power adapter.

Establish Serial Communication Between LBox and
Workstation

I used Minicom on my laptop to establish the serial
connection to the LBox. The details can be found in
this
FAQ. Once Minicom was configured properly, I reset the LBox using
the reset button, located near the board edge, kitty corner from the
serial port header. Then, the Minicom window on my laptop
spewed out the LBox startup messages. These could be useful
subsequently, so I pasted them to an editor on the laptop
for subsequent printout.

When the startup messages were finished, I was presented
with the command prompt. I then investigated the
system to determine what's available. For example,
examining /bin showed both Busybox and Tinylogin were
present. That suggested a small project to update Busybox to
the recent 1.0 version, which has incorporated the
Tinylogin functionality. Other things worth noting:

Certainly you could replace a standard PLC, but would want a sbc that interfaced conveniently (e.g. via optically isolated relays) to the field wiring. Real-Time Linux would be a good idea, so you could truly seize control of the hardware and have a handle on latencies and the interrupt structure. The Real-Time Linux variants already perform control applications. Personally, I would concoct a control language that consisted of concurrent state machines. Such languages have been used before in the commercial sector - as an alternative to relay ladder logic.

Wondering: Does this series apply pretty well to the really really cheap linux appliances? I'm thinking of the LinkSys WRT54G wireless router and NSLU2 network storage, both linux boxes around $70 new (or less) with people hacking the linux installs.

Cheap Linux appliances might be a good route to go. However, you do want the capability to reflash the OS unless you just want to work at the application level. And if you do have the capability to reflash the OS, you need to be able to reflash yet again if your first attempt was a kernel that won't boot. Further, if it is possible to accidentally blow away the boot loader, you want a way to reconstitute the bootloader. These capabilities are typical of well supported SBC's, but not necessarily of the cheap Linux appliances. The latter may have Linux developer/hacker lists where you could ask appropriate questions.

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